1. Field of the Invention
The present invention relates to a lithium secondary battery, particularly to an improvement in a nonaqueous electrolyte in a lithium secondary battery using an electrode formed by depositing a thin film composed of active material capable of lithium storage and release on a current collector.
2. Related Art
In recent years, lithium secondary batteries have been actively developed. In the lithium secondary batteries, the battery characteristics thereof, such as charge-discharge voltage, charge-discharge cycle life characteristics, and storage characteristics, depend largely on the type of an electrode active material used therein.
We have found that an electrode formed by depositing a thin film of active material capable of lithium storage and release, such as an amorphous or microcrystalline silicon thin film, on a current collector exhibits high charge-discharge capacities and superior charge-discharge cycle characteristics. In such an electrode, the thin film is divided into columns by gaps formed therein in a manner to extend in its thickness direction and the bottoms of the columnar portions are adhered to the current collector. In the electrode having such a structure, spaces are formed around the columnar portions, and stress caused by expansion and shrinkage during charge-discharge cycle is relaxed by the spaces to suppress the stress that causes separation of the thin film of active material from the current collector. Therefore, superior charge-discharge cycle characteristics can be obtained.
However, in the lithium secondary battery having such an electrode, the relationship between a nonaqueous electrolyte and charge-discharge cycle characteristics has not been sufficiently investigated.